This invention relates to hydrocarbon fuel processing, and more particularly to an improved shift reactor and its operation with a supported catalyst used therein. More particularly still, the invention relates to an improved shift reactor and its operation with a supported catalyst, for processing hydrogen-rich gas streams, as for use in fuel cells.
Fuel cell power plants that utilize a fuel cell stack for producing electricity from a hydrocarbon fuel are well known. In order for the hydrocarbon fuel to be useful in the fuel cell stack""s operation, it must first be converted to a hydrogen-rich stream. Hydrocarbon fuels that are used by the fuel cell stack pass through a reforming process (reformer) to create a process gas having an increased hydrogen content that is introduced into the fuel cell stack. The resultant process gas contains primarily water, hydrogen, carbon dioxide, and carbon monoxide. The process gas has about 10% carbon monoxide (CO) upon exit from the reformer.
Anode electrodes, which form part of the fuel cell stack, can be xe2x80x9cpoisonedxe2x80x9d by a high level of carbon monoxide. Thus, it is necessary to reduce the level of CO in the process gas, prior to flowing the process gas to the fuel cell stack. This is typically done by passing the process gas through a water gas shift (WGS) converter, or shift reactor, and possibly additional reactors, such as a selective oxidizer, prior to flowing the process gas to the fuel cell stack. The shift reactor also increases the yield of hydrogen in the process gas.
Shift reactors for reducing the CO content of process gas are well known, and typically comprise a chamber having an inlet for entry of the process gas into the chamber, an outlet downstream of the inlet for exit of effluent from the chamber, and a catalytic reaction zone between the inlet and the outlet. The catalytic reaction zone typically contains a catalyst, or catalyst composition, for converting at least a portion of the carbon monoxide in the process gas into carbon dioxide. In operation, a shift reactor carries out an exothermic shift conversion reaction represented by the following equation:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(1)
The reaction (1) between the CO and water concurrently reduces the CO content and increases the CO2 and H2 content of the process gas. The generation of additional hydrogen from this reaction is advantageous to the power plant inasmuch as hydrogen is consumed at the fuel cell anode to produce power. A discussion of one such shift reactor, or converter, is contained in PCT Application U.S. 97/08334 for xe2x80x9cShift Converterxe2x80x9d, published on 27 Nov. 1997 as WO 97/44123. In the shift converter of that application, a catalyst bed contains a catalyst composition of copper and zinc oxide, or copper, zinc oxide, and alumina. Such catalyst composition is further disclosed in U.S. Pat. No. 4,308,176 to Kristiansen, and has been used for a number of years to promote the shift reaction in the shift reactors associated with fuel cell power plants. However, reactors using these catalyst compositions have the limitation that they must be purged with a flow of hydrogen to initially reduce them, and steps must be taken subsequent to operation to prevent significant oxidation or exposure to oxygen. In fact, the required reaction does not work, or occur, unless the catalyst is reduced. Exposure of these catalyst compositions to oxygen is, or may be, detrimental to the catalyst. This is because the catalyst is self-heating in the presence of oxygen, and it can easily heat itself to the point where catalyst particles will sinter, and thus lose surface area and decrease activity. This need to provide a reducing atmosphere and to minimize the possibility of oxygen leaks to the catalyst with a special shutdown purge and the maintenance of an inert atmosphere during shutdown, results in additional hardware and process control considerations that add to the complexity and cost of the fuel cell power plant system, particularly with regard to the shift reactor.
Recent studies show that cerium oxide, or xe2x80x9cceriaxe2x80x9d (CeO2), as well as other metal oxides, can be used as a support in combination with a noble metal or similar type catalyst to promote the shift reaction, eliminate the requirement that the catalyst be reduced, and provide a catalyst that is more oxygen tolerant than the prior catalysts. However, use of such support-promoted catalysts in a conventional manner in existing shift reactors may fail to provide the level of activity for the shift reaction to be useful in a reactor of a reasonable size in the intended operating environment. This is the case for the present two-stage adiabatic shift reactors, with the first stage (high temperature) being the relatively smaller, and the second stage (low temperature) being relatively larger. This is due to equilibrium limitations and low catalyst activity at the low temperatures of this second stage of the reactor. Collectively, an unreasonably large catalyst bed(s) and/or an additional selective oxidizer would be required, in order to reduce the CO to an acceptable level. This would be particularly burdensome in the instance of mobile fuel cell power plants, such as used in vehicles, where space and weight are at a premium.
A recent study has revealed that the removal of carbon monoxide may be enhanced by an oxygen-assisted water gas shift reaction over supported copper catalysts, however such copper catalysts suffer from the limitations discussed above.
It is thus an object of the present invention to provide a shift reactor of reduced spatial volume for effectively reducing the amount of carbon monoxide in a process gas stream, as for a fuel cell.
It is a further object to provide a method of operating a shift reactor in a manner and with such materials as to reduce the spatial volume required by such shift reactor to effect a desired reduction in the amount of carbon monoxide in a process gas stream.
It is a still further object of the invention to provide a shift reactor having the above properties and a tolerance for the presence of oxygen therein, thereby to minimize or eliminate the need for hydrogen reduction.
It is an even further object to provide a shift reactor of relatively reduced size with variable O2 addition to enhance CO reduction for variable CO inputs.
A shift reactor for reducing the amount of carbon monoxide in a process gas, as for a fuel cell power plant, adds a limited quantity of oxygen to the reactor, to provide a further reaction of the carbon monoxide in addition to the water gas shift reaction. The further, or additional, reaction is believed to be an oxidation reaction, though may additionally or alternatively be another reaction such as a surface intermediate elimination reaction, or the like. The oxidation reaction is represented by the following equation:
CO+1/2O2xe2x86x92CO2.xe2x80x83xe2x80x83(2)
The oxygen added is less than about 2.0 mol %, being in the range of about 0.01 to 2.0 mol %, and most typically about 0.2 mol %, or less. The shift reactor includes a reaction chamber, an inlet to the chamber for receiving the process gas and oxygen, an outlet downstream of the inlets for exit of effluent from the chamber, and a catalytic reaction zone between the inlets and the outlet. A catalyst bed, or combination of beds, makes up the catalytic reaction zone, and is selected, in combination with the addition of oxygen, to provide effective removal of carbon monoxide in a spatially-efficient manner without requiring hydrogen reduction and/or purging of the catalytic reaction zone. The catalyst bed comprises a catalytic material deposited on a promoted support. The catalyst is selected from the list of metals including the noble metals platinum, palladium, rhodium, rhenium, indium, silver, gold, osmium, and ruthenium, and the non-noble metals of chromium, manganese, iron, cobalt and nickel. The noble metals of platinum, palladium, rhodium and/or gold are preferred. The promoted support has desirable oxygen storage capacity, and includes at least metal oxides, alone or in combination, and may additionally include silica and/or alumina in combination with the metal oxides. A preferred combination of metal oxides includes ceria and zirconia.
The use of a metal catalyst, preferably of noble metal, a promoted support having desirable oxygen storage capacity, and a small, variable amount of oxygen added to the catalytic reaction zone, yield both a water gas shift reaction and an oxidation reaction which efficiently reduce the carbon monoxide content in the process gas while economizing on the volumetric requirements and complexity of the shift reactor. Moreover, the operation of the shift reactor with these supported catalysts obviates the prior requirements for pre-reducing the catalyst, providing a special post shutdown purge, and maintaining an inert atmosphere during shutdown.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.